Why join?

“We are full members of the EU and we will go on resisting any attempt to set up a rival to Nato”.

He said Britain had always “been concerned about unnecessarily duplicating what we already have in Nato”.

He added that: “We will go on being committed to the security of the European continent … We are not going to back out of our commitment to keeping Europe secure, but we don’t want to see unnecessary bureaucracy at the EU level when we have got it in Nato”.

The remaining 27 EU leaders met in the Slovak capital without the UK in the first such format since the Brexit referendum in June.

Proposals for what Germany has called an EU “defence union” and others have called an "EU army" included the creation of an EU military HQ, with medical aid and logistics capabilities, that would command EU crisis missions.

The operations, such as the naval anti-pirate mission, Atalanta, in the Gulf of Aden, are currently commanded by individual member states.

The proposals, endorsed by France and Germany and by the European Commission, also included joint EU defence budgets, shared military satellite surveillance, and joint procurement of high-tech equipment, such as drones.

UK exit negotiations are expected to start in early 2017 and to last at least two years, with the UK retaining its full rights in the EU Council during that period.

The talks are likely to centre on EU migrants and Britain’s access to the single market, but risk being bedevilled by side issues.

For his part, Andrew Duff, an expert at the European Policy Centre, a think tank in Brussels, and a former British MEP for the Liberal party, said the UK overestimated its unilateral military power.

“One understands more and more why Britain was just no good at being in the EU: It can't defend itself yet will block plans for European army”, he said on Twitter.

Steven Blockmans, a Belgian scholar of EU affairs at the Centre for European Policy Studies, another Brussels think tank, said the UK would "hamper [its] own exit deal by vetoing" EU defence plans.